1. Field of the Invention
This invention relates to bias tees used with radio frequency (RF) and microwave frequency signals in general and more particularly to a miniature wideband bias tee that has a small package size and that can be manufactured at low cost.
2. Description of the Related Art
Bias tees are used in various devices. A bias tee is used with radio frequency and microwave frequency signals to couple a direct current (DC) voltage onto a line used for alternating current (AC or RF) signals. A bias tee can also separate a combined RF and DC signal into separate RF and DC signals. The bias tee can remove the DC component of a composite signal in order to isolate a RF component.
Referring to FIG. 1, a schematic diagram of a bias tee 20 is shown. Bias tee 20 has three ports, DC, RF and RF+DC. A low frequency or DC signal is applied to port DC. A high frequency or RF signal is applied to port RF. A combined signal results at port RF+DC. Bias tee 20 has a RF choke or Inductor L1 and a DC blocking capacitor C. Inductor L1 has one end connected to port DC and another end connected to a node N1. Node N1 joins a DC blocking capacitor C with port RF+DC. The other end of capacitor C is connected to port RF. Inductor L1 can be a wire wound on a ferrite core. The parasitic capacitance of the inductor is shown as capacitor C1. The loss of the ferrite core and the resistance of the wire are shown as resistor R1.
For good performance at low frequencies, the inductance L should be large. Unfortunately, when the inductance is large, the parasitic capacitance is also large and the parasitic resistance low. The result is that the electrical performance of the bias tee is poor at high frequencies.
In order to increase the bandwidth performance of bias tee 20 over a larger frequency range, a second inductor L2 in series can be added. Referring to FIG. 2, a schematic diagram of a wideband bias tee 30 is shown. Wideband bias tee 30 is similar to bias tee 20 except that inductor L2 has been added. Inductor L2 has one end connected to port DC and another end connected to node N2. Inductor L2 can be a wire winding wound on a ferrite core. One end of inductor L1 is connected to node N2. The parasitic capacitance of the inductors are shown as capacitors C1 and C2. The loss of the ferrite cores and the resistance of the wires are shown as resistors R1 and R2. Inductor L1 is selected to be large enough for proper low frequency operation. Inductor L2 is selected to be small for high frequency operation. Since inductor L2 has a small value, the parasitic capacitance C2 is small and the parasitic resistance R2 is high. Therefore, bias tee 30 has good performance at both high and low frequencies.
Bias tees have been packaged using multiple ferrite cores mounted on a printed circuit board with lead frames. Unfortunately, the multiple ferrite cores take up excessive space when mounted on a printed circuit board. The mounting of the cores, side by side, results in a large package. A typical bias tee of the prior art has dimensions of 1.26 inches in length by 0.44 inches in width by 0.39 inches in height. The mounting of the cores, windings and lead frame are manual operations that are difficult to automate. It is desirable, in order to reduce cost, to automate as much of the assembly process as possible.
While bias tees have been used, they have suffered from being too large, expensive, difficult to assemble and not easily manufactured using automated equipment. A current unmet need exists for a wideband bias tee that has a smaller size, can be assembled at a low cost and that can be manufactured using automated equipment.